Filters are used in a variety of electronic circuits to filter out certain frequency components of a signal while letting other frequency components pass. For example, in communication circuits filters may be used to block frequency components outside a frequency band or part of a frequency band used for communication and to be processed by further circuits.
As highly selective bandpass filters in communication circuits and devices, surface acoustic wave (SAW) or bulk acoustic wave (BAW) technologies are frequently used. Conventional filters of such types use acoustic resonators consisting of a piezoelectric material and electrodes. In BAW resonators, the piezoelectric material is sandwiched between two electrodes (e.g. top and bottom electrodes), and acoustic waves propagate through the bulk of the piezoelectric material between the electrodes (e.g. in a vertical direction, which may be referred to asz-direction). SAW resonators utilize interdigitated electrode structures on the surface of the piezoelectric material, and acoustic waves propagate along the surface of the piezoelectric (e.g. within the x-y plane).
Leakage of acoustic waves in acoustic resonators limits the quality factor of the resonator which is defined as a ratio of energy stored in the resonator and energy lost during a resonator cycle. Minimization of acoustic losses is therefore an important factor in acoustic resonator technology and design. In BAW resonators, for example, acoustic leakage can have vertical and lateral components. These loss components can be localized in an area between the electrodes, to the surrounding substrate. A filter, which is made from those resonators will therefore experience such leakage based loss mechanisms.
Furthermore, in some implementations of resonators boundary conditions at resonator edges cause the appearance of so-called spurious mode waves in a BAW resonator which result in passband ripples in a corresponding filter curve of a filter implemented using the BAW resonators. Such spurious mode waves may also cause energy leakage.
When energy is leaking from the resonator, the leaking acoustic waves may also impact adjacent resonators and interact with them in an unwanted way. This may cause uncontrolled acoustic coupling between resonators and may lead to decreased performance of filters. Generally, acoustic energy leaking from the resonator may impact other acoustic resonator devices that are located in the vicinity of the resonator.